Field of the Invention
This invention relates generally to printed circuit boards, and more particularly to an improved process for mounting active electronic components in relation to such boards.
Conventional electronic components such as resistors and capacitors having wire leads are normally mounted on the surface of printed circuit boards, with the leads threaded through holes in the boards. Other surface-mounted devices ("SMDs") are leadless, and may be either tubular or rectangular in cross-section, and have metallic caps on the ends in lieu of wire leads. Those of rectangular form are commonly referred to as "chips." SMD and chip components may comprise resistors, capacitors, diodes and inductors. Typical power handling capability is about 1/8 of a watt for resistors, and a range of 1.0 picofarads to 0.2 microfarad in capacitors.
FIG. 1 depicts a layered printed circuit board assembly showing metal prior art means for installing SMD components on the outer surfaces of printed circuit boards. FIG. 1A depicts a typical SMD component 4, shown as having electrically conductive end caps 6 and 8 for electrical connection to the component in lieu of wire leads. The standard length of a component of this type is 0.130 inch; the length of a "chip" component is slightly less; that is, about 0.126 inch.
Electronic components can also be packaged in a "cordwood" configuration; that is, oriented in parallel either vertically or horizontally, and as close together as thermal and other characteristics will allow. Another well-known form of packaging is by the use of a "honeycomb" of pre-molded or pre-drilled spacers having apertures or openings for receiving and spacing components such as resistors and capacitors.
An automatic component mounter for SMDs has been disclosed that provides for mounting MELF components beneath the surface of the printed circuit board. "MELF" is an acronymic term coined by a Japanese manufacturer for "metal electrode face," and signifies SMD components which have conductive caps on each end in lieu of wire leads. As indicated by FIG. 2, MELF components 10 are inserted into over-size holes 12A, 12B and 12C formed in the base material 14 of a double-sided printed circuit board 16. The components are secured in the holes 12A, 12B and 12C by the application of an epoxy resin 18 which fills the space between the walls of the holes and the sides of the MELF components 10. The caps are then attached by solder 20 to the printed wiring. It will be noted that the holes 12 are preformed in the base material before final assembly by either drilling or molding. The structure and essentially comprises the "honeycomb" described in the foregoing in that the MELF components are inserted into and secured in the preformed compartments. ("Japan unveils below-the-surface-mount technology." Electronic Packaging and Production magazine, May 1987. )